1. Field of the Invention
The present invention relates to an operational amplifier circuit offering a fixed slew rate (SR).
2. Description of Related Art
Operational amplifiers are broadly used. FIG. 1 is an equivalent block diagram of a conventional operational amplifier and a load capacitor.
Referring to FIG. 1, the circuit structure includes an operational amplifier 50 and an output switch 52. An external load circuit is connected to the switch 52. In a load equivalent mode, the load circuit has an equivalent load capacitor 54. When the switch 52 receives a control signal and accordingly is turned on, the operational amplifier 50 starts to charge/discharge the output terminal AOUT. If the operational amplifier 50 charges the output terminal AOUT for a long time, a large peak current will be generated at the instant when the switch 52 is turned on, which will result in the problem of electromagnetic interference (EMI).
The output terminal of an operational amplifier circuit is usually connected to a compensation capacitor, and the capacitance of the compensation capacitor is Cc. When the switch 52 of the operational amplifier is turned on and accordingly enters an output state, the charging time of the compensation capacitor has a slew rate (SR). The SR and the charging time T satisfy the relationship of SR=I/C=ΔV/T. Different charging time will result in different SR. When the operational amplifier 50 charges/discharges the load circuit, an instantaneous large peak current will be generated, and the SR will be changed. As a result, the EMI problem will be produced.
The issue of SR should be taken into consideration when the operational amplifier 50 is designed.